Code is already registered for other domain

I bought my first domain a few days ago with Ionos, and i created a my first web page. I registered the theme, and downloaded the demo version and all was running correctly.
Yesterday a bought a new domain because i wanted to change the name, and as my blog was just the demo version, i didnt have the time to edit nothing i decided that it would be easier to just delete the first webpage and create a new web with the new domain leaving the first one empty as i dont want to use that domain anymore.
So i did this and today when im trying to install the theme i bought, i receive this message “Code is already registered for other domain”.
But the first page dont exist anymore because i deleted it, and i created this new one, and i want the theme to be in this new web page.
The first domain is not deleted, its just “not in use” because ive paid for a year of permanency but its not in use and it doesnt have any working page. Also i deleted the files that the hosting had about it.

So i dont know what to do now. I read that is the Author who needs to change the code itself, im not an expert and i dont understand nothing about codes or API or things like that so please try to explain as simple as posible to me if you know how to fix this!

Thank you VERY MUCH!

Contact the theme author for the domain change

Hi,

for solving purchase code or item license issue will need to get support from the item author. They (author) are the right team to help. you can contact item author through item comments page or author profile page or best option is through item support page.

How to contact an author to get support or for any technical query:

Thanks